This invention relates generally to circuits for protecting single phase induction motors from excessively high rotor winding temperatures brought about by excessive motor loads.
More specifically, the invention relates to the use of the balancing properties of a resistance bridge circuit to compare the motor start winding resistance, which is a function of its temperature, against that of a reference resistor. Numerous motor protection circuits for protection of induction motors against excessively high temperatures are, of course, well known in the art. Characteristically such prior art circuits employ temperature sensing elements, such as thermistors, to sense rotor winding temperature directly. Such temperature sensors are subject to failure, whereby several sensors are often employed by motor protection circuits to provide adequate back-up in case of a failure of one of the sensors.
By means of my invention, the use of temperature sensors in a motor protection circuit is avoided.